1. Technical Field
The present invention relates generally to semiconductor technology and more specifically to reducing the number of steps in forming MirrorBit(copyright) Flash memory.
2. Background Art
Various types of memories have been developed in the past as electronic memory media for computers and similar systems. Such memories include electrically erasable programmable read only memory (EEPROM) and electrically programmable read only memory (EPROM). Each type of memory had advantages and disadvantages. EEPROM can be easily erased without extra exterior equipment but with reduced data storage density, lower speed, and higher cost. EPROM, in contrast, is less expensive and has greater density but lacks erasability.
A newer type of memory called xe2x80x9cFlashxe2x80x9d EEPROM, or Flash memory, has become extremely popular because it combines the advantages of the high density and low cost of EPROM with the electrical erasability of EEPROM. Flash memory can be rewritten and can hold its contents without power. It is used in many portable electronic products, such as cell phone, portable computers, voice recorders, etc. as well as in many larger electronic systems, such as cars, planes, industrial control systems, etc.
In Flash memory, bits of information are programmed individually as in the older types of memory, such as dynamic random access memory (DRAM) and static random access memory (SRAM) memory chips. However, in DRAMs and SRAMs where individual bits can be erased one at a time, Flash memory must currently be erased in fixed multi-bit blocks or sectors.
Conventionally, Flash memory is constructed of many Flash memory cells where a single bit is stored in each memory cell and the cells are programmed by hot electron injection and erased by Fowler-Nordheim tunneling. However, increased market demand has driven the development of Flash memory cells to increase both the speed and the density. Newer Flash memory cells have been developed that allow more than a single bit to be stored in each cell.
One memory cell structure involves the storage of more than one level of charge to be stored in a memory cell with each level representative of a bit. This structure is referred to as a multi-level storage (MLS) architecture. Unfortunately, this structure inherently requires a great deal of precision in both programming and reading the differences in the levels to be able to distinguish the bits. If a memory cell using the MLS architecture is overcharged, even by a small amount, the only way to correct the bit error would be to erase the memory cell and totally reprogram the memory cell. The need in the MLS architecture to precisely control the amount of charge in a memory cell while programming also makes the technology slower and the data less reliable. It also takes longer to access or xe2x80x9creadxe2x80x9d precise amounts of charge. Thus, both speed and reliability are sacrificed in order to improve memory cell density.
An even newer technology allowing multiple bits to be stored in a single cell is known as xe2x80x9cMirrorBit(copyright)xe2x80x9d Flash memory has been developed. In this technology, a memory cell is essentially split into two identical (mirrored) parts, each of which is formulated for storing one of two independent bits. Each MirrorBit Flash memory cell, like a traditional Flash cell, has a gate with a source and a drain. However, unlike a traditional Flash cell in which the source is always connected to an electrical source and the drain is always connected to an electrical drain, each MirrorBit Flash memory cell can have the connections of the source and drain reversed during operation to permit the storing of two bits.
The MirrorBit Flash memory cell has a semiconductor substrate with implanted conductive bitlines. A multilayer storage layer, referred to as a xe2x80x9ccharge-trapping dielectric layerxe2x80x9d, is formed over the semiconductor substrate. The charge-trapping dielectric layer can generally be composed of three separate layers: a first insulating layer, a charge-trapping layer, and a second insulating layer. Wordlines are formed over the charge-trapping dielectric layer perpendicular to the bitlines. Programming circuitry controls two bits per cell by applying a signal to the wordline, which acts as a control gate, and changing bitline connections such that one bit is stored by source and drain being connected in one arrangement and a complementary bit is stored by the source and drain being interchanged in another arrangement.
Programming of the cell is accomplished in one direction and reading is accomplished in a direction opposite that in which it is programmed.
Major problems in the MirrorBit(copyright) architecture can occur during the process for removing the anti-reflective coating (ARC) layers used to pattern core wordlines and peripheral gates. Because the ARC removal process attacks materials used in the charge-trapping dielectric layer, it can allow the formation of conductive silicides in the areas between the bitlines, which can result in short-circuiting the bitlines. It can also cause charges to leak out of the charge-trapping dielectric layer resulting in the loss of important information.
In order to prevent this from happening, a number of additional production steps were required in which exposed materials were protected from attack by the deposition of additional mask layers. While avoiding the problems associated with the ARC removal, the additional steps require increased time, expense, and complexity.
A faster, less expensive, solution to the ARC removal problem that reduces the number of required steps has been long sought but has long eluded those skilled in the art.
The present invention provides a method of manufacturing an integrated circuit with a semiconductor substrate having bitlines under a charge-trapping layer over a core region and a gate insulator layer over a periphery region. A wordline-gate layer, a hard mask, and a first photoresist are deposited and patterned over the core region while covering the periphery region. After removing the first photoresist, wordlines are formed from the wordline-gate layer in the core region. An anti-reflective coating and a second photoresist are deposited and patterned over the periphery region and covering the core region. The anti-reflective coating is removable without damaging the charge-trapping layer. After removing the second photoresist and the anti-reflective coating layer, gates are formed from the wordline-gate layer in the periphery region and the integrated circuit completed. While avoiding the problems associated with the ARC layer removal, the production steps require less time, are less expensive, and make for a simpler process than the prior art.